Answer
Use the Save As Template mechanism: start a new record and fill in the values you want as defaults, but don't save it yet — instead, right-click the line, choose Edit > Save As Template, and give it a title. Then right-click again, choose Edit > New From Template > Set Default Template, and select the template you just saved. From then on, creating a new record of that type pre-populates it with the values from that default template, giving you the same effect as an explicit default-value setting.